In various optical apparatuses such as a spectrometer, an image sensor having a plurality of light receiving elements is used. For example, Patent Document 1 noted below discloses a spectroscope including a photodiode array detector which is an exemplary image sensor. The spectroscope converts an output signal from each of the light receiving elements of the photodiode array detector with an A/D converter.
In each of the light receiving elements of the image sensor of this kind, charges are accumulated in accordance with the amount of received light, and an analog signal corresponding to the charge amount is converted into a digital signal by an A/D converter. By such a conversion process being sequentially executed at fast speed for each of the plurality of light receiving elements, one output value corresponding to the received light amount per light receiving element is acquired. Based on the output values, received light amounts at respective wavelengths are acquired as spectrum data.
FIG. 7 is a timing chart for describing a conventional mode in converting output signals from a plurality of light receiving elements with an A/D converter. In this example, a description will be given of the case where analog signals from n-pieces (where n is a natural number) of light receiving elements are converted into digital signals with an A/D converter.
As shown in FIG. 7, the waveform of an analog signal output from each of the light receiving elements appears as a rectangular wave with a mild rising edge, due to the influence of an image sensor or the time constant of the circuit. Accordingly, the analog signal output from each of the light receiving elements is converted by the A/D converter at a certain timing Tn (where n is a natural number) where the waveform fully rises, and thereafter the process transits to the next light receiving element.
By such a process being sequentially executed at timings T1 to Tn corresponding to respective light receiving elements, one scan operation ends. That is, in one scan operation, one output value is acquired for each of the light receiving elements. During the analysis, by such a scan operation being repeatedly executed, an output value for each of the light receiving elements is obtained at a predetermined cycle.